Fast growth of the pervasive computing and handheld/communication industry has generated exploding demand for high capacity nonvolatile solid-state data storage devices. Current technology like flash memory has several drawbacks such as slow access speed, limited endurance, and the integration difficulty. Flash memory (NAND or NOR) also faces significant scaling problems.
Resistive sense memories are promising candidates for future nonvolatile and universal memory by storing data bits as either a high or low resistance state. One such memory, MRAM, features non-volatility, fast writing/reading speed, almost unlimited programming endurance and zero standby power. The basic component of MRAM is a magnetic tunneling junction (MTJ). MRAM switches the MTJ resistance by using a current induced magnetic field to switch the magnetization of MTJ. As the MTJ size shrinks, the switching magnetic field amplitude increases and the switching variation becomes more severe.
An MRAM write mechanism called Spin-Transfer Torque RAM (STRAM) uses a (bidirectional) current through the MTJ to realize the resistance switching. The switching mechanism of STRAM is constrained locally and STRAM is believed to have a better scaling property than the conventional MRAM.
However, a number of yield-limiting factors should be overcome before STRAM enters the production stage. One concern in traditional STRAM design is that the switching current through the STRAM and transistor is asymmetric depending on the direction of the switching current. This asymmetric switching current can cause reduced reliability of the STRAM.